1. Field of the Invention
The present invention relates to a white light emitting device and, more particularly, to a white light emitting device which uses a wavelength-converting film made of europium-silicate to emit high quality white light.
2. Description of the Related Art
Recently, applications for Light Emitting Diodes (LEDs) have been expanding. High-luminance white light emitting devices have been expanded in their applications ranging from backlights for liquid crystal displays, traffic lights to general illumination. For general illumination, the light emitting devices need to have high luminance and be available at affordable price, which is determined by the technology that enables mass production.
A representative conventional white light emitting device is realized by combining a GaN blue LED and a YAG-based yellow phosphor. The blue light emitted from the blue LED excites the yellow phosphor to emit white light. The mixture of the blue light and the yellow light is perceived as white light by an observer. This is the most widely used configuration, in which the yellow phosphor has high efficiency of near 90% and the blue LED has high efficiency as well, enabling fabrication of a high-luminance product through an easy process. On the other hand, it is difficult to obtain a uniform color from individual products and color changes may occur according to the temperature of the surroundings.
FIG. 1 illustrates a conventional white light emitting device 50. As shown in FIG. 1, a blue LED 4 made of a gallium nitride-based semiconductor is mounted on a mount 5 disposed on a metal stem 3 that forms a reflecting cup 10. The blue LED 4 has one electrode connected to a lead frame 2 by a bonding wire and the other electrode connected to another lead frame 1 by another bonding wire 6. The reflecting cup 10 is filled with an inner encapsulant 8. The lead frames 1 and 2 are sealed by an outer encapsulant 9. A phosphor 11 contained in the inner encapsulant 8 absorbs the blue light emitted from the blue LED 4 to produce a different wavelength of light. For example, the YAG-based yellow phosphor absorbs blue light to emit yellow light. The mixture of the blue light and the yellow light is perceived as white light by an observer, as a result.
However, as the phosphor has to be contained in the inner encapsulant, the white light emitting device 50 has a large size and it is difficult to obtain a uniform color if the phosphor is not distributed evenly inside the inner encapsulant. In addition, combination of the blue LED 4 and the yellow TAG-based phosphor yields low color reproducibility.
A white light emitting device can also be realized by combining a blue LED, a green LED and a red LED. This method reduces the conversion loss of the phosphor and achieves high color reproducibility, but is expensive. With this white light emitting device, destruction of even one chip hinders production of white light.
Furthermore, an UV LED can be combined with red/green/blue phosphor to realize a white light emitting device. This white light emitting device is manufactured through an easy process but has low color reproducibility with a limited level of luminance due to the low efficiency of the red phosphor at below 40%.